1. Field of the Invention
This invention broadly relates to AC induction motors, and deals more particularly with controlling the delivery of AC power to the motor from a variable frequency power supply to avoid excess current at lower frequencies.
2. Description of the Related Art
Electrical AC induction motors are often used for higher power electrical motor applications where polyphase electrical power is available. The phase differences between the 3 phases of the polyphase electrical power source creates a rotating electromagnetic field in the motor. This rotating magnetic field induces a current in the conductors in the rotor which in turn sets up a counterbalancing magnetic field that causes the rotor to turn in the direction the field is rotating. Induction motors are produced in standardized frame sizes making them nearly interchangeable between manufacturers and widely used throughout industry for many applications. AC induction motors using a single phase power source are also available in which the rotating magnetic field is produced using any of several techniques.
Induction motors are designed to operate at a synchronous speed using a fixed frequency/fixed voltage AC power supply, such as 60 cycles or 400 cycles per second. Induction motors operate with a small amount of “slip percentage” from the supply frequency, typically 5 to 10%, so that if the load increases, the motor slows slightly, and the slip percentage increases, which in turn increases the motor torque to compensate for the increased torque load. FIG. 1 illustrates the concept of the slip percentage, in which the motor torque 20 is plotted as a function of motor speed. A typical load is shown at 26 along with plots of the maximum and minimum loads 22, 24, respectively. The synchronous frequency of the motor is shown as fo. From the torque curve 20 shown in FIG. 1, it can be seen that the area of slip percentage 28 increases with higher loads and slower speeds. Thus, in effect, synchronous induction motors utilize a simple, self regulating system when operating from a fixed frequency, AC power supply.
Using a fixed frequency power supply, induction motors are self regulating for both speed and power, making them suitable for use in a wide range of applications, including aircraft applications such as fuel pumps and hydraulic pumps. In some applications, however, such as in commercial and military aircraft, induction motors must be powered by variable frequency power supplies. Problems are presented by the use of variable frequency power supplies due to the non-linear relationship between the amounts of current drawn by the motor at different AC frequencies. This nonlinear relationship is illustrated in FIG. 2 which depicts motor torque curves 30 at four differing power supply frequencies. A plot of the motor current is shown at 32 as a function of motor speed. Also, the maximum and minimum loads are shown at 22 and 24 respectively. From FIG. 2, it can be seen that the available slip percentage 28 decreases with motor speed while the current drawn by the motor increases nonlinearly. In fact, motor current can increase by the inverse ratio of the frequency-squared with decreasing motor speed, resulting in overheating of induction motors at the lower frequencies.
At variable frequencies over a range greater than 1.7:1, induction motors are designed for optimum performance at the highest frequency but exhibits inferior performance characteristics at lower frequencies. Thus, where it is impractical to use motor controllers, the selection of a motor for a given application represents the best compromise possible since most induction motors are standard, off-the-shelf commercial products intended for a wide range of use. The selection of standardized induction motors for aircraft applications is particularly difficult since the variable frequency ratio can be as high as 2.2 from about 360-800 Hz. As a result, motors intended to operate at 800 Hz have high current and slip issues when operating at 360 Hz.
One remedy for the problems discussed above involves the use of solid state motor controllers which rectify the variable AC frequency and convert it to a constant frequency. This is accomplished by rectifying the variable frequency power into DC and then synthesizing AC power that matches the motor requirements. This solution, however, is relatively expensive because of the number of components that are required and thus is not suitable for solving the problem in smaller induction motors since the cost of the controller can exceed that of the motor itself.
Another solution to the problem consists of providing a cooling system for the motor which draws away excess heat generated by high current levels at lower speeds. Such cooling systems not only add to cost, but are bulky and add undesirable weight in aircraft applications.
Accordingly, there is a need for a power control for induction motors that avoids the problems discussed above, which is simple in construction and is also both cost effective and light weight. The present invention is directed towards satisfying this need.